Engineering plastics such as ABS/PC, PC, PBT, Noryl, nylon and the like have been used as various home appliances and electric materials. In case of using such products, a flame retardancy of the products is considered importantly when a fire occurs. Among them, a drip phenomenon that a portion of the molten products is dropped with flame is particularly problematic. To prevent the drip phenomenon, namely to provide an anti-drip property, various methods have been developed. One of them is that PTFE is used as an anti-drip flame-retarding additive.
In industrial sites, a method of simply kneading PTFE powders into engineering plastics has mainly been used. However, in case of direct use of PTFE to the products, dispersability is poor. Therefore, many methods for improving the dispersability of PTFE have been developed. To improve the dispersability of PTFE, the powders containing PTFE have been developed.
EP-A-0,166,187 discloses a PTFE containing powder composition. The powder is obtained by mixing poly(tetrafluoroethylene) dispersoids, and a graft polymer latex, for example a latex of acrylonitrile-butadiene-styrene graft polymer, salting out, filtering and then drying the mixture. However, the powder obtained by co-precipitation or co-salting out embraces a problem in that it has a tendency of self-adhesion. In particular, when the PTFE content is high, for example, the PTFE is contained in an amount of 25% by weight or more based on the total weight of the composition, the condition becomes worse. As a result, since the powder does not flow freely, it is difficult to handle and store.
U.S. Pat. No. 5,804,654 discloses a tetrafluoroethylene polymer containing powder obtained by emulsion polymerizing a mixture of acrylonitrile and styrene monomer in the presence of a tetrafluoroethylene polymer latex and encapsulating the tetrafluoroethylene polymer. In the above patent, since the tetrafluoroethylene polymer containing powder can be obtained in the form of a freely-flowing powder, there is no self-adhesion of powders. However, such a process is so complicate, since it comprises emulsion polymerization of tetrafluoroethylene to firstly obtain a PTFE latex, addition of monomers to the obtained PTFE latex, polymerization of a resultant mixture to secondly obtain a latex product, and salting out the obtained latex product to enlarge particles so as to use the resulting latex in the form of powder.
Therefore, there is still a need for a method that can more simply obtain a PTFE-containing powder.